diff --git a/cat_win/tests/mocks/std.py b/cat_win/tests/mocks/std.py
index fe44412f..5427b60c 100644
--- a/cat_win/tests/mocks/std.py
+++ b/cat_win/tests/mocks/std.py
@@ -5,6 +5,24 @@
 import io
 # import re
 
+
+class OSAttyDefGen:
+    """
+    generate a function to replace os.isatty()
+    """
+
+    @staticmethod
+    def get_def(mapping: dict):
+        """
+        return a function that returns the mapping
+        """
+        def isatty(no: int, *args, **kwargs) -> bool:
+            """
+            return the mapped values
+            """
+            return mapping.get(no, False)
+        return isatty
+
 # ansi_escape_8bit = re.compile(r'''
 #     (?: # either 7-bit C1, two bytes, ESC Fe (omitting CSI)
 #         \x1B
@@ -38,16 +56,6 @@ def fileno(self) -> int:
         return 1
 
 
-class StdOutMockIsAtty(io.StringIO):
-    """
-    mock for atty stdout stream
-    """
-    closed: bool = False
-
-    def isatty(self) -> bool:
-        return True
-
-
 class StdInMockIter:
     """
     mock for stdin stream iterable
@@ -60,6 +68,9 @@ def __init__(self, mock: object) -> None:
     # def __iter__(self):
     #     return self
 
+    def fileno(self) -> int:
+        return 0
+
     def __next__(self) -> str:
         if self.index < len(self.splitted_input_value)-1:
             self.index += 1
diff --git a/cat_win/tests/test_full.py b/cat_win/tests/test_full.py
index 958698ca..bed21e8f 100644
--- a/cat_win/tests/test_full.py
+++ b/cat_win/tests/test_full.py
@@ -3,7 +3,7 @@
 import os
 
 from cat_win import cat
-from cat_win.tests.mocks.std import StdInMock, StdOutMock, StdOutMockIsAtty
+from cat_win.tests.mocks.std import StdInMock, StdOutMock
 from cat_win.util.argparser import ArgParser
 from cat_win.util.holder import Holder
 # import sys
@@ -205,7 +205,7 @@ def test_cat_output_full_show_wordcount(self):
     @patch('cat_win.cat.sys.argv',
            ['<CAT>', test_file_path, 'trunc=0:0', '--UNIQUE', '--b64', '-?'])
     def test_cat_output_suggestions(self):
-        with patch('cat_win.cat.sys.stderr', new=StdOutMockIsAtty()) as fake_out:
+        with patch('cat_win.cat.sys.stderr', new=StdOutMock()) as fake_out:
             cat.main()
             self.assertIn("Unknown argument: '--UNIQUE'", fake_out.getvalue())
             self.assertIn("Did you mean --unique", fake_out.getvalue())